A typical ultrasonic imaging system makes use of one or more piezoelectric transducers which act as the source (actuator) of the ultrasonic beam or signal, and which often also serve to sense the reflected signal (sensor). An electrical pulse generated by an electronic control module is converted to an ultrasonic pulse by the transducer/actuator in the probe. The probe is in contact with the body, and the ultrasonic pulse is transmitted therein. The pulse is then absorbed by body tissues or reflected to different degrees from the boundaries between body tissues. The reflections reach the transducer/sensor at different times, which vary with the distance to the tissue boundaries. The reflections also have different energies, due the different acoustic impedances of the tissues, as well as absorption by the intervening tissues. The transducer/sensor converts the reflections into a weak electrical signal, which contains information that can be processed into an image of the body.
A great variety of ultrasonic transducers are presently in use or under development. Shapes and sizes vary widely in order to meet special needs. Focusing by electronic or mechanical means, or some combination thereof, can be used to produce and steer a narrow ultrasonic beam of desired focal length. Likewise, mechanical and electronic focusing can be used to sense the reflections from a particular direction and distance. Phased transducer arrays of various configurations have been employed to achieve particular focusing properties, under electronic control. (The term "phased array" is taken from radar technology, in which the phase relationships of signals from multiple antennae are processed electronically to improve resolution and sensitivity.) The acquired signal is then converted into an image using analog or, depending on cost and technological considerations, digital processing.
Good resolution of ultrasound images is important for medical applications. Some limits to resolution are fundamental to the physics of wave propagation (for example, acoustic shadows and reverberations, and geometric artifacts) and are best dealt with by educating the user, or by appropriate image processing algorithms. Other factors affecting resolution involve transducers and electronic instrumentation (such as axial and lateral resolution, and dynamic range) and are susceptible to improvement.
Axial resolution can be limited in part by the wavelength of the ultrasonic signal ("ultrasound" simply designates sound waves of a frequency above the audible range, with wavelengths of millimeters or less). Absorption of ultrasonic energy by body tissues tends to restrict the useful depth of field to about 200 wavelengths, due to attenuation of the signal. Thus resolution can be improved by use of shorter wavelengths (higher frequencies) but this also implies a shallower depth of field.
For a simple system with a single element and spherical or parabolic focusing, the lateral resolution is limited by the aperture of the transducer. Larger apertures provide greater resolution but shallower depth of field. The size of the transducer element or elements also can limit the resolution, since the detected signal will be known to originate from a given transducer but not any particular location on that transducer.
The dynamic range of the instrument determines the useful number of gray scale levels in the image. Most commercial transducers use piezoelectric crystal elements or other materials (e.g. plastics or polymers) both as actuators which produce the ultrasonic pulse, and as sensors which detect the reflected signal. The physics and engineering of piezoelectric sensors are relatively well understood. The sensitivity of a simple piezoelectric sensor, such as a small block of quartz, can be greatly improved by use of a more complicated geometry, the "piezoelectric bimorph" shape. The bimorph has been used since 1930 in microphones and phonograph needle assemblies, but various design considerations such as high cost and fragility preclude its use in ultrasound transducers.
An alternative means of sensing small deflections or increments of motion is the optical lever. Optical levers have proven to be effective in routine measurements of extremely small deflections, of less than 0.01 nanometer, in atomic force microscopy (AFM). This measurement strategy can be implemented in robust ultrasound transducers at low cost, with great flexibility in design.